Physical and structural characterization of bis-acryl composite resin

During the preparation of fixed prosthesis (including individual bridges and crowns) it is important to select the materials that have the best features and properties to predict a successful clinical treatment. The objective of this study was to determine if the chemical and structural characteristics could cause to increase the fracture resistance, we used four bis-acryl resins Luxatemp, Protemp, Structur and Telio. Three-points bending by Flexural test were performed in ten bars and they were carried out to compare with Anova test. In addition, the bis-acryl resins were analyzed by scanning electron microscopy, to analyze microstructure and morphology and the molecular structure were performed by Infrared Spectroscopy through Attenuated Total Reflectance. A higher flexural strength was found in Luxatemp and Structur with, no significant differences between this study groups. Regarding Protemp and Telio, these study groups showed a lower flexural strength when were compared with Luxatemp and Structur. These results corroborate SEM and ATR analysis because Luxatemp sample showed a regular size particle on the surface and chemically presents a long cross-linkage polymer chain. The presence of CO3, SiO2 and N–H groups as a fillers particle interacting with OH groups cause a higher flexural strength compared with another groups.

www.nature.com/scientificreports/which could cause a better fracture resistance by flexural test on three-points, using four types of bis-acryls (Luxatemp, Protemp, Structur Premium, and Telio).

Mechanical properties
The characterization of the mechanical properties of the four bis-acrylic materials were performed by obtaining the values of the fracture resistance by flexural test (Table 1).The values showed that Luxatemp has the higher flexural strength (211.44 ± 23.31 MPa), followed by Structur (207.33 ± 17.26 MPa).Furthermore, Protemp (173.57± 14.10 MPa) and Telio (152.00 ± 25.94 MPa) showed less flexural strength in comparison to the Luxatemp and Structur (Table 1).In Table 2, ANOVA test showed statistically significant differences between study groups regarding to flexural strength of four bis-acryl resins analyzed in this study with a p value of 0.0001.A post Hoc Tukey analysis was developed to identify between which groups these differences occurred.As can be seen in Table 3, the statistically significant differences were observed when compared Luxatemp with Protemp and Telio (p = 0.001 and p = 0.0001) respectively.These differences were also observed when comparing Structur group with Protemp and Telio (p = 0.004 and p = 0.0001) respectively.However, this mechanical behavior is related to the different ultrastructural surface morphology and the filler material as showed differences in the four bis-acrylic materials by SEM.

SEM analysis
The analysis of the ultrastructural surface morphology of the four bis-acrylic materials showed an irregular surface with different particle size (Fig. 1).For Luxatemp bis-acrylic SEM analysis showed a micro particles approximately of 5 µm to 10 µm distributed on the irregular surface (Fig. 1a and b).The Structur bis-acrylic showed a similar irregular surface topography with different shapes and overlapping particles that has a smaller than 5 µm in size (Fig. 1c and d).The Telio bis-acrylic images revealed some fine wrinkles observed on the surface with greater amounts of filler particles with size of 1-6 µm, which were more densely and attached to the surface (Fig. 1e and f).Finally, the Protemp bis-acrylic showed an irregular surface with craters and round hole-likes with size of 20-40 µm and particles that surround them (Fig. 1g and h).

FTIR analysis
The molecular structure and the functional groups presented in the four bis-acrylic materials were analyzed by FTIR spectroscopy (Fig. 2).The vibration bands at 3600-3400 cm −1 are related to the N-H groups in the Structur and Telio materials.In the range of the 2970-2840 cm −1 is related to asymmetric and symmetric stretching of methylene group CH 2 vibrations in the Structur, Telio, Protemp, and Luxatemp materials.Moreover, the vibrations at 1740-1710 cm −1 , 1640-1600 cm −1 , and a double band in the region of 1510-1450 cm −1 is attributed to

Discussion
Recently, several manufactures and modifications of new materials have been introduced, but there is not much research on the mechanical performance and its relationship with the ultrastructural characteristics and chemical compounds to provide them resistance available of these bis-acryl composites.Our results were similar to the last research of Luxatemp and other three bis-acrylic (Provipont, Protemp 3 Garant, and Integrity) resins demonstrated significantly superior flexural strength over traditional methacrylate resins.Such results suggest that this behavior is due to dysfunctional and capacity of cross-linking with another monomer chain imparting strength and toughness to the materials 5 .There was a condition to be considered, as the time to exposure to water because in previous reports it has been demonstrated that a highly significant decrease in flexural strength was seen for the bis-acrylic materials (Protemp 4, Cooltemp natural, and Luxatemp Fluorescence) from 24 h to 8 days, but there was no difference at 8 days 10 , it is possibly attributable to the excessive water uptake, which can promote breakdown causing a filler matrix debonding, this can spread polymer chains apart and facilitate slippage between chains 11 .At the other extreme, temporary crowns were produced by a direct fabrication methods, by using bis-acrylic composites (Structur Premium, Takilon, System c & b II, and Acrytemp) and they were subjected to sterilization wrapping at room temperature to be measured and determined the fracture strength, which showed that Structur Premium obtained the highest values in comparison to the other bis-acrylic materials, and they concluded that different chemical composition could be the cause for difference in fracture strength being difunctional materials, thus, they are capable of cross-linking with another monomer chain 12 .
Nevertheless, our study demonstrated that the images obtained of Protemp sample by SEM, showed holelikes distributed over the surface and craters-likes which it was not possible to establish a calculating in the depth, hence, we suggest that this decreases the fracture resistance and possibly in a clinical condition act as a bacterial reservoir.
Telio sample showed the larger particle size (> 3 µm), this provides for crack propagation within the sample to occur primarily through the polymer matrix between filler particles 13 .The manufacturer indicates an inorganic filler is the cause of failure to the flexural testing, and it is not able to stop the fracture.
The surface of Luxatemp and Structur were similar in other study moreover the failure analysis was performed by biaxial flexure test showing an increase in value of Structur after 15 and 30 days of storage compared with 7 days 14 .They assumed that water storage yielded a more brittle behavior, this was due to the crack propagation within bis-acryl materials that occurs primarily through the polymer matrix between filler particles 13 , and it may interfere in the mechanical performance principality for the water sorption and swelling that could affects the overall polymer strength 15 .
Inorganic Filler particles related to Silica (Si) were observed in FTIR spectra at about 1090-1020 cm −1 and a possible interaction at 820-830 cm −1 with the O-H 16,17 , in Protemp sample showed a weak signal at 954 cm −1 corresponding to the Si-O-Z cross-linkage 18 , these zirconium particles present in Protemp sample to have shown www.nature.com/scientificreports/ a decrease in the fracture strength by flexural test on three-points.Our outcome was different from other prior studies [19][20][21][22] , where Protemp shows higher values using different.
The presence a band showed at 3380 cm −1 and 3660 cm −1 in Structur and Telio samples respectively confirm the typical absorptions of OH group, which has been the cause for the decrease in the fracture resistance as it has been reported in other study 11 .
According to the findings of our study, Luxatemp was the bis-acryl resin with the highest flexural strength although Structur showed a similar flexural strength with significant difference when was compared to Protemp and Telio.These outcomes are related to the filling particles, mainly zirconium and silica, which could cause the average pore size and irregular surface of the polymer chains as indicated by SEM.Furthermore, the absorption of OH can interact with the filler matrix promoting, consequently, the decrease in the polymer strength by interactions with CO 3 , SiO 2 and N-H groups.Nonetheless, it is necessary to carry out more studies with conditions that emulate the clinical and oral environment.

Preparation of specimens
This study was designed to characterize and compare four commercial bis-acryl composite resins: (1) Luxatemp (DMG), ( 2) Protemp (3M ESPE), (3) Structur Premium (VOCO), and ( 4) Telio (Ivoclar Vivadent) using SEM, Infra-red and mechanical analysis.Paste and catalyst pastes of each bis-acryl composite resin were mixed with dispensing guns and automix syringes and placed into molds according to norm 27 ANSI/ADA No. 27.All samples used for the assay previously rinsed with 70% ethanol and stored in double distilled water by 14 days at 37 °C.

Properties characterization of bis-acrylic specimens
The ultrastructural surface morphology of the four bis-acrylic materials was made by a SEM microscopy (JSM-6060LV), using an acceleration voltage at 5 kV with secondary electrons were performed.All samples for SEM analysis were sputtering coated (EMS 559) with a golden thin film.On the other hand, the physicochemical properties of the four bis-acrylic materials were evaluated with a FTIR spectrophotometer (Pekin Elmer) using a diamond ZnSe crystal plate.Forty scans for each spectrum were collected acquiring 1 scan/s at 5 cm −1 resolution in the wavenumber range of 500-4500 cm −1 .Lastly the fracture resistance by flexural test on three-points mechanical assay were performed ten specimens from each bis-acrylic material were used under the norm 27 ANSI/ADA No. 27.The flexure strength was performed on a computer-controlled universal testing machine (CMS Metrology, Model WDW-5Y, Querétaro, Mexico) by means of the three-point bend test.Each specimen was mounted with its edges equidistant from the midline of the holder.The load was applied at a crosshead speed of 0.75 mm/min until its fracture.The data were collected in Newtons and converted to megapascals (MPa) according to the following equation: Flexure strength = 3FL/(2BH 2 ), where the maximum load was represented by F; L was the distance between supports (mm); B was the width of the specimen (mm); and H was the height (mm).

Statistical analyses
The flexural strength results are expressed as means and standard deviation and were analyzed with the statistical software SPSS Version 26 through descriptive and inferential statistics using Shapiro Wilk, and ANOVA test with post-hoc Tukey.Statistical significance was set at p ˂ 0.05.
www.nature.com/scientificreports/C=O of Poly (methyl methacrylate) for the four bis-acrylic materials.The C-O stretching vibration peaks were at 1260-1230 cm −1 , and 1160 cm −1 .The asymmetric Si-O-Si stretching vibrations was observed at 1090 cm −1 , and 1020 cm −1 meanwhile the O-H deformation vibration peak was at 820-830 cm −1 when the hydroxyl group was bonded to a silicon atom.Finally, there were different identified weak bands between 1460 and 1350 cm −1 assigned to pigment materials, stabilizers, and filler particles.

Table 1 .
Descriptive results of study groups and Shapiro Wilk test.SD: Standard Deviation, p value ≤ 0.05: significant differences, SW: Shapiro Wilk.
Sum of squares df quadratic mean F value p value